1. Field of the Invention
This invention relates generally to tractors for moving equipment within passages.
2. Description of the Related Art
The art of moving equipment through vertical, inclined, and horizontal passages plays an important role in many industries, such as the petroleum, mining, and communications industries. In the petroleum industry, for example, it is often required to move drilling, intervention, well completion, and other forms of equipment within boreholes drilled into the earth.
One method for moving equipment within a borehole is to use rotary drilling equipment. In traditional rotary drilling, vertical and inclined boreholes are commonly drilled by the attachment of a rotary drill bit and/or other equipment (collectively, the “Bottom Hole Assembly” or BHA) to the end of a rigid drill string. The drill string is typically constructed of a series of connected links of drill pipe that extends between ground surface equipment and the BHA. A passage is drilled as the drill string and drill bit are together lowered into the earth. A drilling fluid, such as drilling mud, is pumped from the ground surface equipment through an interior flow channel of the drill string to the drill bit. The drilling fluid is used to cool and lubricate the bit, and only recently for drilling to remove debris and rock chips from the borehole, which are created by the drilling process. The drilling fluid returns to the surface, carrying the cuttings and debris, through the annular space between the outer surface of the drill pipe and the inner surface of the borehole. As the drill string is lowered or raised within the borehole, it is necessary to continually add or remove links of drill pipe at the surface, at significant time and cost.
Another method of moving equipment within a borehole involves the use of a downhole tool, such as a tractor, capable of gripping onto the borehole and thrusting both itself and other equipment through it. Such tools can be attached to rigid drill strings, but can also be used in conjunction with coiled tubing equipment. Coiled tubing equipment includes a non-rigid, compliant tube, referred to herein as “coiled tubing,” through which operating fluid is delivered to the tool. The operating fluid provides hydraulic power to propel the tool and the equipment and, in drilling applications, to lubricate the drill bit. The operating fluid also can provide the power for gripping the borehole. In comparison to rotary equipment, the use of coiled tubing equipment in conjunction with a tractor should be generally less expensive, easier to use, less time consuming to employ, and should provide more control of speed and downhole loads. Also, a tractor, which thrusts itself within the passage and pushes and pulls adjoining equipment and coiled tubing, should move more easily through inclined or horizontal boreholes. In addition, due to its greater compliance and flexibility, the coiled tubing permits the tractor to perform much sharper turns in the passage than rotary equipment.
A tractor can be utilized for drilling boreholes as well as many other applications, such as well completion and production work for producing oil from an oil well, pipeline installation and maintenance, laying and movement of communication lines, well logging activities, washing and acidizing of sands and solids, retrieval of tools and debris, and the like.
One type of tractor comprises an elongated body securable to the lower end of a drill string. The body can comprise one or more connected shafts in addition to a control assembly housing or valve system. This tractor includes at least one anchor or gripper assembly adapted to grip the inner surface of the passage. When the gripper assembly is actuated, hydraulic power from operating fluid supplied to the tractor via the drill string can be used to force the body axially through the passage. The gripper assembly is longitudinally movably engaged with the tractor body, so that the body and drill string can move axially through the passage while the gripper assembly grips the passage surface. A gripper assembly can transmit axial and even torsional loads from the tractor body to the borehole wall. Several highly effective designs for a fluid-actuated gripper assembly are disclosed in U.S. Pat. No. 6,464,003, which is incorporated by reference herein. In one design, the gripper assembly includes a plurality of flexible toes that bend radially outward to grip onto the passage surface by the interaction of ramps and rollers.
Some tractors have two or more sets of gripper assemblies, which permits the tractor to move continuously within the passage. Forward longitudinal motion (unless otherwise indicated, the terms “longitudinal” and “axial” are herein used interchangeably and refer to the longitudinal axis of the tractor body) is achieved by powering the tractor body forward with respect to an actuated first gripper assembly (a “power stroke” with respect to the first gripper assembly), and simultaneously moving a retracted second gripper assembly forward with respect to the tractor body (a “reset stroke” of the second gripper assembly). At the completion of the power stroke with respect to the first gripper assembly, the second gripper assembly is actuated and the first gripper assembly is retracted. Then, the tractor body is powered forward while the second gripper assembly is actuated (a power stroke with respect to the second gripper assembly), and the retracted first gripper assembly executes a reset stroke. At the completion of these respective strokes, the first gripper assembly is actuated and the second gripper assembly is retracted. The cycle is then repeated. Thus, each gripper assembly operates in a cycle of actuation, power stroke, retraction, and reset stroke, resulting in longitudinal motion of the tractor. A number of highly effective tractor designs utilizing this configuration are disclosed in U.S. Pat. No. 6,003,606 to Moore et al., which discloses several embodiments of a tractor known as the “Puller-Thruster Downhole Tool;” U.S. Pat. No. 6,241,031 to Beaufort et al., which discloses an “Electro-Hydraulically Controlled Tractor;” and U.S. Pat. No. 6,347,674 to Bloom et al., which discloses an “Electrically Sequenced Tractor” (“EST”).
The power required for actuating the gripper assemblies, longitudinally thrusting the tractor body during power strokes, and longitudinally resetting the gripper assemblies during reset strokes may be provided by pressurized operating fluid delivered to the tractor via the drill string—either a rotary drill string or coiled tubing. For example, the aforementioned Puller-Thruster Downhole Assembly includes inflatable engagement bladders and uses hydraulic power from the operating fluid to inflate and radially expand the bladders so that they grip the passage surface. Hydraulic power is also used to move forward cylindrical pistons residing within sets of propulsion cylinders slidably engaged with the tractor body. Each set of cylinders is secured with respect to a bladder, so that the cylinders and bladder move together longitudinally. Each piston is longitudinally fixed with respect to the tractor body. When a bladder is inflated to grip onto the passage wall, operating fluid is directed to the proximal side of the pistons in the set of cylinders secured to the inflated bladder, to power the pistons forward with respect to the borehole. The forward hydraulic thrust on the pistons results in forward thrust on the entire tractor body. Further, hydraulic power is also used to reset each set of cylinders when their associated bladder is deflated, by directing drilling fluid to the distal side of the pistons within the cylinders.
A tractor can include a valve system for, among other functions, controlling and sequencing the distribution of operating fluid to the tractor's gripper assemblies, thrust chambers, and reset chambers. Some tractors, including several embodiments of the Puller-Thruster Downhole Tool, are all-hydraulic. In other words, they utilize pressure-responsive valves and no electrically controlled valves. One type of pressure-responsive valve shuttles between its various positions based upon the pressure of the operating fluid in various locations of the tractor. In one configuration, a spool valve is exposed on both ends to different fluid chambers or passages. The valve position depends on the relative pressures of the fluid chambers. Fluid having a higher pressure in a first chamber exerts a greater pressure force on the valve than fluid having a lower pressure in a second chamber, forcing the valve to one extreme position. The valve moves to another extreme position when the pressure in the second chamber is greater than the pressure in the first chamber. Another type of pressure-responsive valve is a spring-biased spool valve having at least one end exposed to fluid. The fluid pressure force is directed opposite to the spring force, so that the valve is opened or closed only when the fluid pressure exceeds a threshold value.
Other tractors utilize valves controlled by electrical signals sent from a control system at the ground surface or even on the tractor itself. For example, the aforementioned EST includes both electrically controlled valves and pressure-responsive valves. The electrically controlled valves are controlled by electrical control signals sent from a controller housed within the tractor body. The EST is preferred over all-hydraulic tractors for drilling operations, because electrical control of the valves permits very precise control over important drilling parameters, such as speed, position, and thrust. In contrast, all-hydraulic tractors, including several embodiments of the Puller-Thruster Downhole Tool, are preferred for so-called “intervention” operations. As used herein, “intervention” refers to re-entry into a previously drilled well for the purpose of improving well production, to thereby improve fuel production rates. As wells age, the rate at which fuel can be extracted therefrom diminishes for several reasons. This necessitates the “intervention” of many different types of tools. Hydraulic tractors, as opposed to electrically controlled tractors, are preferred for intervention operations because intervention, as opposed to drilling, does not require precise control of speed or position. The absence of electrically controlled valves makes hydraulic tractors generally less expensive to deploy and operate.
Tractors in combination with coiled tubing equipment are particularly useful for intervention operations because, in many cases, the wells were originally drilled with rotary drilling equipment capable of drilling very deep holes. It is more expensive to bring back the rotary equipment than it is to bring in a coiled tubing unit. However, the coiled tubing unit may not be capable of reaching extended distances within the borehole without the aid of a tractor.
In one known design, exemplified by FIG. 3 of U.S. Pat. No. 6,003,606 (which discloses the Puller-Thruster Downhole Tool), a tractor includes a spool valve whose spool has two main positions. In one main position, the valve directs pressurized fluid to a first gripper and to propulsion chambers of a first set of propulsion cylinders. In this position of the spool, the pressure is permitted to decrease in a second gripper and in reset chambers of a second set of propulsion cylinders. In the other main position, the valve does the opposite—it directs pressurized fluid to the second gripper and propulsion chambers of the second set of cylinders, and permits pressure to decrease in the first gripper and in propulsion chambers of the first set of cylinders. The spool of the valve is piloted by fluid pressure on both ends of the spool. A pair of cycle valves selectively administers high pressure to the ends of the spool. Each cycle valve is in turn piloted by the pressure in the fluid passages to the cylinders and grippers.
The Puller-Thruster all-hydraulic tractor design has proven to be a major advance in the art of tractors for moving equipment within boreholes. However, it operates most effectively within a limited zone of parameters, including the pressure, weight, and density of the operating fluid, the geometry of the tractor components, and the total weight of the equipment that the tractor must pull and/or push. Thus, it is desirable to provide an improved design for a tractor, which will work within a much larger zone of such parameters.
Another prior design consists of a wellbore tractor having wheels that roll along the surface of the well casing. This design is problematic because the wheels do not have the ability to provide significant gripping force to move heavier downhole equipment. Also, the wheels can lose traction in certain conditions, such as in regions including sand.
A typical process of extracting hydrocarbons from the earth involves drilling an underground borehole and then inserting a generally tubular casing in the borehole. In order to access oil reserves from a given underground region through which the well passes, the casing must be opened within that region. In one method, perforation guns are brought to the desired location within the well and then utilized to cut openings through the casing wall and/or the earth formation. Oil is then extracted through the openings in the casing up through the well to the surface for collection. Perforation guns can also be used to penetrate the formation in an “open hole” to access desired oil reserves. An open hole is a borehole without a casing. Perforation guns can be ignited by different means, such as by pressurized operating fluid or electricity provided through electrical lines (“e-lines”). However, the practice of igniting the perforation guns with e-lines poses the risk of a spark leading to explosion and potential loss of life. Thus, it is desirable to fully hydraulic tractors, without e-lines, for operations that involve the use of perforation guns.
Perforation guns are commonly used in conjunction with rotary drilling equipment, due to the large weight of the guns. Long strips of perforation guns can weigh up to 20000 pounds or more. The rotary drilling equipment, consisting of the rigid drill string formed from connected links of drill pipe, has been used because of its ability to absorb the weight in tension. However, the use of rotary equipment is very expensive and time-consuming, due in part to the necessity of assembling and disassembling the portions of drill pipe.
In the prior art, shafts designed for downhole tools used in drilling and intervention applications have been formed from more flexible materials, such as copper beryllium (CuBe). This is because in drilling it is not uncommon to experience sharp turns, and the tool is preferably capable of turning at sharp angles. Also, shafts have been formed with relatively large internal passages for the flow of operating fluid to the valves and other equipment of the BHA. This is because in drilling the operating fluid is typically drilling mud, which often contains larger solids and necessitates a larger flow passage. The drilling mud is preferred because it provides better lubrication to the drill bit and more effectively carries the drill cuttings up through the annulus back to the ground surface.
The shaft of a downhole tool typically must include multiple internal passages (e.g., for fluid to the gripper assemblies, propulsion chambers, and the other downhole equipment) that extend along the shaft length. In the past, such passages have been formed by gun-drilling, which is well known. Unfortunately, it is typically not possible to gun-drill the entire length of the shaft (in most applications, the length of a shaft for a downhole tool can be anywhere in the range of 50 to 168 inches). The distance that a passage can be gun-drilled is limited by (1) the inherent length limitations of known gun-drilling tools, and (2) the limitations imposed by the geometry and material characteristics of the shaft. In the past, it has been necessary to limit the length of gun-drilled passages in shafts of downhole tools to a relatively great degree. This is because the larger internal passage required for drilling mud leaves less room for other fluid passages. This shortage of available “real estate” in the shaft requires higher precision gun-drilling and increases the risk of inadvertent damage to other passages caused by the gun-drilling process. These problems are exacerbated by the fact that the more flexible materials used for the shaft (e.g., CuBe) are softer, more difficult to drill through, and more prone to damage.
The limitations on the length that passages can be gun-drilled have necessitated forming the shafts from a plurality of shaft portions of reduced length. The fluid passages are gun-drilled in each shaft portion, and then the shaft portions are attached to each other. Due in large part to the use of CuBe, shaft portions have been attached together by electron beam welding. Electron beam welding is favored because it maintains the structural integrity of the material and of the fluid passages contained therein. Unfortunately, electron beam welding is a very expensive process. Most conventional welding processes have not been used because they do not facilitate the welding together of thick objects (i.e., the weld does not fuse completely through the objects). In shaft manufacturing for downhole tools, it is necessary to soundly fuse together all of the mating surfaces in order to maintain zero leakage between the various internal fluid passages and to provide structural integrity.